Our team has created backend systems capable of handling millions of users. E.g. Social Calendar, a Facebook app, had approx. 20 million installed users.
Languages & Frameworks: Node.js, J2EE, Python/Django, Ruby on Rails, PHP/Laravel
Relational Databases: Microsoft SQL Server, PostgreSQL, MySQL
Search and NoSQL: Elastic, Solr, MongoDB, Firestore, Dynamo DB
Cache: Redis, Memcached
Cloud Integration: AWS, Azure, Heroku, Firebase
Blockchain: Hedera, Ethereum, Hyperledger Fabric